Aims: To evaluate the relationship between amplitude of accommodation and refractive errors in the peri-presbyopic age group. Materials and Methods: Three hundred and sixteen right eyes of 316 consecutive patients in the age group 35-50 years who attended our outpatient clinic were studied. Emmetropes, hypermetropes and myopes with best-corrected visual acuity of 6/6 J1 in both eyes were included. The amplitude of accommodation (AA) was calculated by measuring the near point of accommodation (NPA). In patients with more than ± 2 diopter sphere correction for distance, the NPA was also measured using appropriate soft contact lenses. Results: There was a statistically significant difference in AA between myopes and hypermetropes ( P <0.005) and between myopes and emmetropes ( P <0.005) in the 35-39 year age group. In the 40- 44 year age group, there was a significant difference in AA between emmetropes and hypermetropes ( P <0.0001), emmetropes and myopes ( P <0.01) and hypermetropes and myopes ( P <0.0001). In patients above 45 years of age there was no significant difference ( P >0.5). Conclusion: Our study showed higher amplitude of accommodation among myopes between 35 and 44 years compared to emmetropes and hypermetropes

Accommodation is the ability of the eye to change the refractive power of the lens to automatically focus on objects at various distances. It is a complex constellation of sensory, neuromuscular and biophysical phenomena by which the overall refracting power of the eye changes rapidly to image objects at different viewing distances clearly on to the retina. [1] The factors that cause presbyopia are still unclear. [2]

The point at which accommodation is maximally exerted is called the near point. Amplitude of accommodation (AA) is the amount of accommodation exerted to move the focus from the far point to the near point. It decreases from childhood to 65 years. [3]

The definition of presbyopia is fluid because there is no standard distance for near work. [4] The age at which patients seek help for presbyopic symptoms vary and it is not unusual to see patients even in their late forties, not wearing presbyopic lenses. This variability could be either real, due to variable preservation of accommodative ability or artifactual. Symptoms of presbyopia itself can be dependent on other factors like the amount of near work done, lighting conditions, corrected distance acuity etc. [5]

It is well known that the myopes seek help for presbyopic symptoms much later than the rest. It could be due to the fact that these individuals remove their glasses for near work; [6] the under correction prescribed for myopes delays presbyopic symptoms; or myopes preserve their accommodation for longer periods. There is also a suggestion that near work and thereby increased accommodative effort or capability is a major factor in the development of simple myopia. [7]

Corrected hypermetropes have a lower effective accommodation compared to emmetropes and will need near addition at a younger age. The converse applies to myopes. This is due to the lower effectiveness of convex lenses for near compared to concave lenses. Hypermetropes are thus more symptomatic earlier than emmetropes or myopes.

To study accommodation and its relationship with refractive errors, McBrien et al looked at the amplitude of accommodation in a group of young university students between 18 and 22 years and found that the amplitude of accommodation was highest in late onset myopes and least in hypermetropes. [8] Schaeffel et al have shown that refractive errors do not affect the dynamics of natural accommodation. [9] To study presbyopia however we thought it would be better to study the accommodative process in the peri-presbyopic age group. There is little data on the actual differences in accommodation that is preserved in various types of refractive errors in this age group. To quantify presbyopia we have used amplitude of accommodation as a measure of accommodative reserve.

Materials and Methods

This was a cross-sectional study. All patients who attended our outpatient department between July 1999 and July 2000 were eligible to be considered for the study provided they fulfilled the inclusion and exclusion criteria detailed below.

An emmetropic eye was defined as one in which the spherical correction is less than or equal to ± 0.25 diopter after undilated retinoscopy and subjective refraction. Hypermetropia was defined as any eye with a spherical correction of more than or equal to + 0.50 diopter. Myopic eyes were those with a spherical correction of more than or equal to - 0.50 diopter. The completed age in years was taken for age calculation. The best-corrected visual acuity was obtained after undilated retinoscopy and subjective refraction. A duochrome test was done for all patients to prevent over or under correction for distance.

A complete ophthalmic examination was done for all patients. All patients were examined before 11:00 a.m. With full distance correction in the trial frame placed at 15 mm from the eye, the near point of accommodation (NPA) was measured, one eye at a time, using an RAF rule. The NPA was measured with the patient trying to read the smallest letter (N5) on the RAF rule target. With the RAF rule in place the target was moved from 50 cm to the point where the last line became slightly blurred. Then the target was slowly pushed back till the last line was just clearly read. This point was taken as the near point of accommodation. No correction for interpupillary distance was attempted because the correction factor was too small to make any significant difference in the dioptric value of amplitude in this age group. If the NPA was more than 50 cm, a +1.00 D spherical lens was added to the trial frame and the near point measured again. The actual NPA was calculated mathematically from this value. Reciprocal of the NPA in metres is the amplitude of accommodation (AA).

To obviate the effect of back vertex distance on amplitude of accommodation, patients with a spherical correction of 2.00DSph or more had measurements taken, using appropriate soft contact lenses also.

Two of the authors (V.S; N.V) took all the measurements. The kappa value for inter-observer agreement for these two examiners was 0.82. This was near perfect agreement.

The right eye of each patient was used for analysis. The data were statistically analyzed by calculating the means and standard deviations of amplitude of accommodation for the entire group and for age-specific groups in emmetropes, hypermetropes and myopes. Student's t test, Pearson's correlation coefficient, 95% confidence interval for the means and regression analysis were also used for statistical analysis.

Results

One hundred and forty-four of 316 patients were males and the right eye of each patient was used for calculation. One hundred and sixty-two eyes were emmetropic, 104 eyes were hypermetropic and 50 were myopic. [Table - 1] gives the distribution of the number of eyes studied, according to age groups and refractive errors.

The mean and confidence interval of the amplitude of accommodation for the different age groups and refractive errors is as shown in [Table - 2]. There was a statistically significant difference in AA between myopes and hypermetropes ( P <0.005) and between myopes and emmetropes ( P <0.005) in the 35-44 year age group. In the 40-44 year age group, there was a significant difference in AA between emmetropes and hypermetropes ( P < 0.0001), emmetropes and myopes ( P <0.01) and hypermetropes and myopes ( P < 0.0001). In patients above 45 years of age there was no significant difference ( P > 0.5).

Discussion

The effect of age on the onset and progression of presbyopic symptoms is well known. The role of refractive error on the onset of presbyopia is poorly understood. We studied patients between 35 and 50 years of age because we thought this would better allow us to detect changes when the accommodative reserve is borderline and presbyopic symptoms are setting in. This would also help us understand the reasons for late presentation of myopes for their presbyopic corrections. The error in measurement of NPA is also reduced, as one needs to move objects over larger distances when there is around 3D of accommodation compared to 10 or more diopters in the younger age group.

We did not find in the literature any study measuring amplitude of accommodation in different groups of refractive errors in the peri-presbyopic age group. However one study looked at near vision AA in patients above 41 years but this did not show any significant difference in AA between the different refractive groups. [11]

The pattern that emerges from our study is as follows. The amplitude of accommodation is highest in myopes and lowest in hypermetropes till the age of 44 years. The AA in emmetropes is in between the two. Though the AA in the emmetropes was more than the hypermetropes in the 35 to 39 year age group it was not statistically different probably because of the smaller number of hypermetropes in this group. After the age of 44 years there seems to be no difference in AA between the three refractive groups.

The convergence of AA depicted in [Figure - 1] suggests that clinically undetectable lenticular changes which start after the age of 40 years may be the primary reason for reduced accommodation; giving credence to the theory that the lens rather than ciliary muscle is primarily responsible for the decrease in AA as one ages. [12] In tropical areas it is well known that cataracts occur in a younger age group and that lenticular sclerosis occurs earlier. [13], [14] This could also be the reason why the amplitude of accommodation is less in the tropical peri-presbyopic population compared to the European population. [15]

One must be cautious in generalizing the results from a hospital-based study. A population-based study would have been ideal.

In conclusion our study showed hitherto unreported higher amplitude of accommodation among myopes in the 35 and 44 year age group compared to emmetropes and hypermetropes. This is probably why myopes develop presbyopic symptoms later. After the age of 44 years the AA converges to similar values in the three refractive groups